Microbiology

Arsenic-Metabolizing Bacteria in Extreme Environments

The health of millions of people around the world is at risk from drinking well water contaminated with arsenic. In general, this arsenic has been found to occur naturally, rather than coming from identifiable point sources of pollution. With increasing evidence that mobilization of arsenic from subterranean geologic formations into the aqueous phase is a microbiological phenomenon, USGS scientists and others have been conducting field and laboratory investigations to isolate arsenic metabolizing bacteria from diverse environments and to identify which specific microbiological mechanisms are responsible for arsenic dissolution. Research in hypeprsaline, alkaline enviroments has been supported in part by the National Aeronautics and Space Administration (NASA) since the studies also may serve as a biogeochemical guide in the search for microbial life on other planets.

In 2005, USGS scientists reported (Oremland et al, 2005) the discovery of bacteria living in an environment which had conditions so extreme that it was assumed to be sterile. This arsenic-metabolizing bacterium, SLAS-1, was found in Searles Lake, California. Located in the Mojave Desert, Searles Lake is about ten times saltier and about 70-times more alkaline than seawater and contains arsenic in concentrations that are unusually high (about 4 mM). The bacteria isolated by the USGS scientists were found to not only tolerate the extreme conditions, but to derive energy from metabolizing arsenic.

In 2008, scientists from the USGS and several universities reported a primitive form of photosynthesis in which bacteria were found to use arsenic instead of water for photosynthesis (Kulp et al, 2008). This anaerobic photosynthetic oxidation of As(III) may have been an essential mechanism for establishing and maintaining the arsenic cycle on the ancient anoxic Earth.

And in December 2010, a NASA scientist and her colleagues from USGS and elsewhere reported (Wolfe-Simon et al, in advance of print in Science Express) a bacterium that can grow by using arsenic instead of phosphorus. They report that the bacterium, strain GFAJ-1 isolated from Mono Lake, California, substitutes arsenic for phosphorus to sustain its growth. Their data show evidence for arsenate in nucleic acids and proteins that normally contain phosphate, a finding that might have profound evolutionary and geochemical significance.